32 Dr. Wentworte Eck on the Saielirtes of Mars. 
a faint object at about 290°, and distant from the limb rather more than 
two diameters of the planet. During the short time I was able to observe it, the 
direction of the motion resembled that of a satellite moving in the direction. in 
which the motion of a double star is reckoned. 
It subsequently appeared that I was mistaken in supposing this to have been 
the satellite; but yet the observation of this star has proved most valuable ; for, 
its position with regard to the planet was similar to that in which the true satellite 
was afterwards seen, whilst its apparent brightness was, as nearly as I could judge, 
the same as that of the satellite itself. Thus, when the star was left behind, by 
the motion of the planet, we had an isolated standard of comparison whereby to 
estimate the brightness of the satellite irrespective of the light of its primary. 
The telescope I employed was a 73-inch by Alvan Clarke, and it is a curious 
coincidence, that the telescope with which the satellites were discovered in America, 
and that with which the outer satellite was first seen in the United Kingdom,were both 
by the same maker. Contrary to my expectation, there was no difficulty in seeing 
the satellite when proper precautions were taken. I used an hexagonal aperture on 
the object glass, and, as an eye piece, a single lens giving a power of 300. The field 
of view was limited to two minutes of arc, and the planet itself placed just outside 
the field. I then turned the hexagonal aperture till the satellite was exactly in the 
centre of the angle formed by the rays; when this was done it only remained to 
read off by the position circle the position of the rays, and adding 30°, there was 
the position of the satellite. But the estimate of distance was little more than 
suess-work, the standard of estimation not being in sight along with the distance 
to be estimated. 
On every occasion I looked for the inner satellite, but looked in vain ; indeed, so 
far as I have been able to ascertain, the inner satellite has not been seen in Europe. 
But, one word with regard to the use of large apertures in such cases. They 
seem to me to be nearly useless, as in practice they are found to illuminate the 
back ground against which the faint object is seen. Of course, if the figure and 
material of the object-glass were perfect, or even as good’as some of the small 
ones, and if it could be relieved from the strain produced by its own weight, then 
the gain would be great ; but, so far as my experience and information go, such is 
not the case. 
As an instance, of the very small aperture with which the satellite could be seen, 
I may mention that I had little difficulty in seeing it steadily, not by glimpses, 
with an equilateral triangular aperture, whose side was 6 inches, giving an area 
of 163 inches, equivalent to a circular aperture of 44 inches diameter, and IT am 
quite confident that it could have been seen with a yet smaller aperture. Still, 
though this aperture may appear small to have shown the satellite, cn loco, it is 
perfectly certain, judging from the comparison hereafter to be mentioned, that less 
than one-third of this aperture, that is to say, less than one-tenth of the area would 
